Electron beam devices, in particular a scanning electron microscope (SEM) or a transmission electron microscope (TEM), are used for examining samples in order to obtain knowledge concerning the properties and behavior of these samples under specific conditions.
In the case of an SEM, an electron beam (also called primary electron beam hereinafter) is generated using a beam generator and focused by a beam guiding system, in particular an objective lens, onto a sample to be examined (also called object hereinafter). Using a deflection device, the primary electron beam is guided in a raster-type fashion over a surface of the sample to be examined. In this case, the electrons of the primary electron beam interact with the material of the sample to be examined. Interaction particles, in particular, arise as a consequence of the interaction. By way of example, electrons are emitted by the sample to be examined (so-called secondary electrons) and electrons of the primary electron beam are backscattered at the sample to be examined (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used for image generation. An imaging of the sample to be examined is thus obtained.
Furthermore, it is known from the prior art to use combination devices for examining samples, in which devices both electrons and ions can be guided onto a sample to be examined. By way of example, it is known to additionally equip an SEM with an ion beam column. Using an ion beam generator arranged in the ion beam column, ions are generated which are used for the preparation of a sample (for example for removing a layer of the sample or for applying material to the sample) or else for imaging purposes. In this case, the SEM serves, in particular, for the observation of the preparation, but also for the further examination of the prepared or unprepared sample.
Furthermore, it is known from the prior art to arrange an aperture unit in an electron beam device. The known aperture unit has an aperture body having an aperture opening extending from a first side of the aperture body to a second side of the aperture body. The aperture opening is configured in such a way that electrons of the primary electron beam can pass through the aperture opening. The known aperture unit can have a plurality of functions. Firstly, it can have the function of aperture delimitation, such that only electrons from a specific beam cone of the primary electron beam pass through the aperture opening. The aperture unit accordingly has an aperture-delimiting effect. Secondly, it serves for example as a pressure stage, which, in the electron beam device, separates a first region having a first pressure (for example a high vacuum region) and a second region having a second pressure (for example an ultra high vacuum region) from one another. Furthermore, the known aperture unit can additionally be embodied as a detector or as part of a detector unit.
With regard to the prior art, reference is made for example to DE 198 28 476 A1 and EP 0 917 178 A1, which are incorporated herein by reference.
FIG. 1 shows an aperture unit 1 known from the prior art, said unit being provided with an aperture body 2 and an aperture opening 3. The aperture unit 1 is arranged in an evacuated beam column (not illustrated) of an electron beam device (not illustrated) and has a first side 4 of the aperture body 2, said first side being directed toward an electron beam generator (not illustrated), and a second side 5 of the aperture body 2, said second side being directed toward an object to be examined (not illustrated). The aperture opening 3 extends from the first side 4 to the second side 5 and is embodied in such a way that electrons of the primary electron beam generated by the electron beam generator can pass through the aperture opening 3 from the direction of the first side 4 in the direction of the second side 5 in order subsequently to be focused onto the object to be examined. The aperture opening 3 is embodied such that it is substantially cylindrical. The axis of symmetry of this cylindrical embodiment basically corresponds to the optical axis of the electron beam device. A central ray 6 of electrons of the primary electron beam runs substantially along the optical axis and passes unhindered through the aperture opening 3.
Electrons which are situated in an outer region of the beam cone of the primary electron beam, however, are masked out of the primary electron beam by the aperture body 2. By way of example, electrons of a first lateral ray 7 and of a second lateral ray 8 impinge on the first side 4 of the aperture body 2 and are masked out of the primary electron beam. When the electrons of the first lateral ray 7 and of the second lateral ray 8 impinge on the first side 4 of the aperture body 2, interaction particles arise, for example in the form of secondary electrons SE and backscattered electrons.
These interact with residual gas 9 (in particular hydrocarbons) which is still situated in the beam column and which has adsorbed, in particular, on a surface of the aperture body 2. Furthermore, the residual gas 9 can also move along the first side 4 of the aperture body 2. The interaction has the effect that said hydrocarbons are chemically altered and deposit on the first side 4 of the aperture body 2 as deposits 10 in the form of solid substances. As an alternative thereto, the electrons of the primary electron beam interact directly with the hydrocarbons, such that the latter are chemically altered and likewise deposit in the form of solid substances on the surface of the aperture body 2. After lengthy irradiation of the first side 4 by electrons of the primary electron beam and depending on the quantity of residual gas 9 present in the beam column, relatively large deposits 10 (contaminations) are formed. Furthermore, the interactions described likewise take place when electrons of the primary electron beam are incident on the deposits 10, such that the effect of deposit formation is further intensified. The geometry of the surface of the aperture body 2 is altered by the deposits 10. Even thin deposits 10 can lead to charges that influence the primary electron beam in an undesirable manner. This leads to undesired disturbances particularly in the case of imagings that are intended to be achieved. Furthermore, the deposits 10 can grow after a certain time of irradiation of the first side 4 of the aperture body 2 with the primary electron beam in such a way that the aperture opening 3 becomes overgrown. This is undesirable.
The effect of deposits also occurs on the second side 5 of the aperture body 2. Residual gas 11 (in particular hydrocarbons) is likewise situated on the second side 5 of the aperture body 2. On the second side 5 of the aperture body 2 in the region of the aperture opening 3 the residual gas 11 impinges on the primary electron beam, in particular on electrons of the primary electron beam which pass through the aperture opening 3 at the edge of the aperture opening 3. These are illustrated in FIG. 1 by a first marginal ray 12 and a second marginal ray 13. These electrons impinge, for example, on the edge region of the aperture opening 3, where they produce secondary electrons, in particular, as a result of interaction with the edge region of the aperture opening 3. Said secondary electrons in turn interact with the residual gas 11, thus resulting in deposits 14 at the aperture opening 3 in the region of the second side 5 of the aperture body 2. As an alternative thereto, the electrons of the primary electron beam interact directly with the residual gas 11, such that the deposits 14 occur. The deposits 14 occur at the edge region of the aperture opening 3 on the second side 5 of the aperture body 2. The above-described depositing process is continued further by constant subsequent diffusion of residual gas 11 at the deposits 14. The deposits 14 grow in the course of time as a result of continued irradiation with electrons of the primary electron beam in such a way that the aperture opening 3 becomes overgrown.
It has been found that the undesired effect of the deposits 14 is particularly great if the aperture unit 1 is used as a pressure stage and the pressure on the second side 5 of the aperture body 2 is higher than the pressure on the first side 4 of the aperture body 2.
Accordingly, it would be desirable to specify an aperture unit and a particle beam device comprising an aperture unit wherein the effect of the overgrowth of an aperture opening is prevented, reduced and/or at least slowed down.